This invention relates to polyhydropolyborates useful as transesterification catalysts in melt polymerization reactions of dihydroxy aromatic compounds with diaryl carbonates. Suitable polyhydropolyborates are those which comprise alkali metal counterions. The invention further relates to a method for the preparation of polycarbonates using these polyhydropolyborates. The method provides a product polycarbonate comprising a lower level of Fries product than is provided by other known methods employing conventional melt transesterification catalysts.
Increasingly, polycarbonate is being prepared by the melt reaction of a diaryl carbonate with a dihydroxy aromatic compound in the presence of a transesterification catalyst, such as sodium hydroxide. In this xe2x80x9cmeltxe2x80x9d process, reactants are introduced into a reactor capable of stirring a viscous polycarbonate melt at temperatures in excess of 300xc2x0 C. Typically, the reaction is run at reduced pressure to facilitate the removal of by-product hydroxy aromatic compound formed as the diaryl carbonate reacts with the dihydroxy aromatic compound and growing polymer chains.
The Fries rearrangement is a ubiquitous side reaction taking place during the preparation of polycarbonate using the melt process. The resultant xe2x80x9cFries productxe2x80x9d serves as a site for branching of the polycarbonate chains thereby affecting flow and other properties of the polycarbonate. Although, a low level of Fries product may be tolerated in the product polycarbonate produced by the melt process, the presence of higher levels of Fries product may negatively impact performance characteristics of the polycarbonate, such as moldability and toughness. Currently, alkali metal hydroxides, such as sodium hydroxide, are employed as catalysts in the preparation of polycarbonate using the melt process. Alkali metal hydroxides, although effective catalysts in terms of rates of conversion of starting materials to product polycarbonate, tend to produce relatively high levels of Fries rearrangement product. Thus, melt polymerization methodology useful for the preparation of polycarbonate in which the formation of Fries product has been minimized represents a long sought goal among those wishing to practice such methodology.
It would be a significant advantage to prepare polycarbonate by a melt polymerization method which provides high rates of polymerization while minimizing the amount of Fries product formation.
The present invention provides a method for the preparation of polycarbonate, said method comprising contacting under melt polymerization conditions at least one diaryl carbonate with at least one dihydroxy aromatic compound in the presence of a transesterification catalyst to afford a polycarbonate, said transesterification catalyst comprising at least one polyhydropolyborate catalyst and at least one co-catalyst.
In one aspect the method of the present invention affords a product polycarbonate having a lower level of Fries rearrangement product than polycarbonate prepared using a conventional melt transesterification catalyst.
The present invention further relates to a method of preparing polycarbonate by the melt reaction of at least one dihydroxy aromatic compound with at least one diaryl carbonate in the presence of at least one transesterification catalyst comprising a polyhydropolyborate catalyst having structure I
(BnHm)(M)pxe2x80x83xe2x80x83I
wherein (BnHm) is a monovalent or divalent anion consisting of boron and hydrogen; n and m are positive integers n having a value greater than or equal to 3, m having a value greater than or equal to n; M is an alkali metal cation; and p is an integer having a value of 1 or 2.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
As used herein the term xe2x80x9cpolycarbonatexe2x80x9d refers to polycarbonates incorporating structural units derived from at least one dihydroxy aromatic compound and includes copolycarbonates and polyester carbonates.
As used herein, the term xe2x80x9cmelt polycarbonatexe2x80x9d refers to a polycarbonate made by a process comprising the transesterification of a diaryl carbonate with a dihydroxy aromatic compound in the presence of a transesterification catalyst.
xe2x80x9cBPAxe2x80x9d is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.
As used herein when describing the instant invention, the term xe2x80x9ctransesterification catalystxe2x80x9d refers to a catalyst system comprising at least one xe2x80x9cprincipal catalystxe2x80x9d and at least one co-catalyst. For Example, in one embodiment of the instant invention diphenyl carbonate and bisphenol A are melt polymerized in the presence of a transesterification catalyst comprising Cs2B12H12 and tetramethylammonium hydroxide, Cs2B12H12 being the principal catalyst and tetramethylammonium hydroxide being the co-catalyst.
xe2x80x9cCatalyst systemxe2x80x9d as used herein refers to the catalyst or catalysts that catalyze the transesterification of the dihydroxy aromatic compound with the diaryl carbonate in the preparation of melt polycarbonate.
xe2x80x9cCatalytically effective amountxe2x80x9d refers to the amount of the catalyst at which catalytic performance is exhibited.
As used herein the term xe2x80x9cFries productxe2x80x9d is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, the Fries product affords carboxy bisphenol A, II, upon complete hydrolysis of the product polycarbonate. 
The terms xe2x80x9cFries productxe2x80x9d and xe2x80x9cFries groupxe2x80x9d are used interchangeably herein.
The terms xe2x80x9cFries reactionxe2x80x9d and xe2x80x9cFries rearrangementxe2x80x9d are used interchangeably herein.
As used herein the term xe2x80x9chydroxy aromatic compoundxe2x80x9d means a phenol, such as phenol, p-cresol or methyl salicylate, comprising a single reactive hydroxy group and is used interchangeably with the term xe2x80x9cphenolic by-productxe2x80x9d.
As used herein the term xe2x80x9caromatic radicalxe2x80x9d refers to a radical having a valence of at least one and comprising at least one aromatic ring. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group, a phenethyl group or a naphthylmethyl group. The term also includes groups comprising both aromatic and cycloaliphatic groups for example 4-cyclopropylphenyl and 1,2,3,4-tetrahydronaphthalen-1-yl.
As used herein the term xe2x80x9caliphatic radicalxe2x80x9d refers to a radical having a valence of at least one and consisting of a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.
As used herein the term xe2x80x9ccycloaliphatic radicalxe2x80x9d refers to a radical having a valance of at least one and comprising an array of atoms which is cyclic but which is not aromatic, and which does not further comprise an aromatic ring. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclopropyl, cyclopentyl cyclohexyl, 2-cyclohexylethy-1-yl, tetrahydrofuranyl and the like.
It should be understood that as used herein, the terms xe2x80x9caliphatic radicalxe2x80x9d, xe2x80x9caromatic radicalxe2x80x9d and xe2x80x9ccycloaliphatic radicalxe2x80x9d include both substituted and unsubstituted embodiments of said radicals. For example, a radical comprising the cyclohexane ring structure alone may be regarded as an unsubstituted cycloaliphatic radical and an analogous six membered ring structure comprising a methyl group (CH3) may be taken as an example of a substituted cycloaliphatic radical. Typical substituents according to the present invention for the substituted forms of aliphatic, aromatic, and cycloaliphatic radicals, include C1-C20 alkyl, C3-C20 cycloalkyl, C4-C20 aryl, halogen, hydroxy, carbonyl, nitro, cyano, C1-C20 alkoxy, C1-C20 alkoxycarbonyl, and the like.
It should be understood that the terms xe2x80x9cmmHgxe2x80x9d and xe2x80x9ctorrxe2x80x9d are used interchangeably herein as units of pressure.
It has been discovered that the use of transesterification catalysts comprising polyhydropolyborates as the principal catalyst in the melt transesterification reaction of a dihydroxy aromatic compound such as bisphenol A with a diaryl carbonate such as diphenyl carbonate, provides a product polycarbonate having a reduced level of Fries rearrangement product relative to polycarbonates prepared with a conventional transesterification catalyst comprising sodium hydroxide as the principal catalyst. This reduction in the amount of Fries rearrangement is highly desirable in that it results in increased ductility of the product polycarbonate and avoids uncontrolled branching which may occur at sites of Fries rearrangement. Uncontrolled branching may limit the utility of the product polycarbonate by reducing the ductility of the product polycarbonate. Transesterification catalysts comprising polyhydropolyborates used according to the method of the present invention, produced less Fries rearrangement product than did transesterification catalysts comprising alkali metal hydroxides, such as sodium hydroxide.
In one embodiment, the present invention provides a transesterification catalyst comprising at least one polyhydropolyborate as the principal catalyst for the production of polycarbonate under melt polymerization conditions wherein the polycarbonate has a number average molecular weight, Mn of at least about 8000 daltons and a reduced content of Fries products relative to a polycarbonate of comparable molecular weight prepared using a transesterification catalyst comprising an alkali metal hydroxide as the principal catalyst. In particular, it is desirable to have Fries product of less than 3000 ppm, preferably less than 2000 ppm, more preferably less than 1000 ppm, even more preferably less than 500 ppm.
The present invention relates to polyhydropolyborates useful as catalysts in the melt polymerization of dihydroxy aromatic compounds with diaryl carbonates. Polyhydropolyborates having structure I are effective melt polymerization catalysts and are exemplified by Na2B12H12, K2B12H12, Cs2B12H12, Na2B10H10, K2B10H10, Cs2B10H10, NaB11H14, KB11H14, CsB11H14, NaB3H8, KB3H8, CsB3H8, and the like. Catalysts having structure I are commercially available or may be prepared by known methods. See for Example U.S. Pat. Nos. 3,555,261 and 3,328,134, and Inorganic Chemistry Vol. 6 (No.6) pages 1196-8 (1967).
Dihydroxy aromatic compounds which are useful in preparing polycarbonates according to the method of the present invention may be represented by the general formula III 
wherein R1 is independently at each occurrence a halogen atom, nitro group, cyano group, C1-C20 alkyl group, C4-C20 cycloalkyl group, or C6-C20 aryl group; q and s are independently integers 0-3; and W is a bond, an oxygen atom, a sulfur atom, a SO2 group, a C1-C20 aliphatic radical, a C6-C20 aromatic radical, a C6-C20 cycloaliphatic radical or the group 
wherein R2 and R3 are independently a hydrogen atom, C1-C20 alkyl group, C4-C20 cycloalkyl group, or C4-C20 aryl group; or R2 and R3 together form a C4-C20 cycloaliphatic ring which is optionally substituted by one or more C1-C20 alkyl, C6-C20 aryl, C5-C21 aralkyl, C5-C20 cycloalkyl groups or a combination thereof.
Suitable bisphenols III according to the method of the present invention include bisphenol A; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 2,2-bis(3-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-isopropylphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane; and 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane.
In one aspect of the present invention, the diaryl carbonate used according to the method of the present invention has structure IV 
wherein R4 is independently at each occurrence a halogen atom, nitro group, cyano group, C1-C20 alkyl group, C1-C20 alkoxycarbonyl group, C4-C20 cycloalkyl group, or C6-C20 aryl group; and t and v are independently integers 0-5.
Diaryl carbonates IV suitable for use according to the method of the present invention are illustrated by diphenyl carbonate, bis(4-methylphenyl)carbonate, bis(4-chlorophenyl)carbonate, bis(4-fluorophenyl)carbonate, bis(2-chlorophenyl)carbonate, bis(2,4-difluorophenyl)carbonate, bis(4-nitrophenyl)carbonate, bis(2-nitrophenyl)carbonate, and bis(methyl salicyl)carbonate (CAS No. 82091-12-1).
In one embodiment, structural units of the product polycarbonate derived from the dihydroxy aromatic compound are comprised entirely of structural units derived from bisphenol A, and structural units derived from the diaryl carbonate are derived entirely from diphenyl carbonate.
Optionally, one or more branching agents may be included during the melt polymerization reaction according to the method of the present invention as a means of effecting the controlled branching of the product polycarbonate as is sometimes desirable in applications, such as in blow molding of beverage bottles, requiring a high degree level of melt strength. Suitable branching agents include 1,1,1-tris(4-hydroxyphenyl)ethane (THPE) and 1,3,5-trihydroxybenzene.
In the process of the present invention, an endcapping agent may optionally be used. Suitable endcapping agents include hydroxy aromatic compounds such as phenol, p-tert-butylphenol, p-cumylphenol, cardanol and the like.
When an endcapping agent is employed said endcapping agent is preferably used in an amount corresponding to between about 0.001 and about 0.10 moles, preferably about 0.01 to about 0.08 moles per mole of the dihydroxy aromatic compound employed.
The polyhydropolyborate catalyst having structure I is employed in an amount corresponding to between about 1xc3x9710xe2x88x928 and 2.5xc3x9710xe2x88x924, preferably between about 1xc3x9710xe2x88x927 and 2.5xc3x9710xe2x88x925 moles of catalyst per mole of dihydroxy aromatic compound employed. When the amount of catalyst employed is less than 1xc3x9710xe2x88x928 mole of catalyst per mole of dihydroxy aromatic compound employed, reaction rates may be reduced to such an extent that no appreciable molecular weight gain is observed. Generally, it is preferred that the number average molecular weight (Mn) of the product polycarbonate be at least about 8000 daltons. When the amount of catalyst is in excess of about 2.5xc3x9710xe2x88x924 moles per mole of dihydroxy aromatic compound employed, rates of Fries rearrangement may be excessive and high levels of uncontrolled branching may occur.
The co-catalyst used according to the method of the present invention is a quaternary ammonium compound, a quaternary phosphonium compound, or a mixture thereof.
Examples of suitable quaternary ammonium compounds include, but are not limited to ammonium hydroxides having alkyl groups, aryl groups and alkaryl groups, such as tetramethylammonium hydroxide (TMAH) and tetrabutylammonium hydroxide (TBAH). Suitable phosphonium compounds include, but are not limited to tetraethylphosphonium hydroxide, tetrabutylphosphonium hydroxide, and tetrabutylphosphonium acetate.
The co-catalyst is preferably used in amounts of from about 1xc3x9710xe2x88x922 to about 1xc3x9710xe2x88x926, preferably about 1xc3x9710xe2x88x922 to about 1xc3x9710xe2x88x925 moles per mole of dihydroxy aromatic compound.
In some instances the reaction mixture may further comprise a metal hydroxide, for example, an alkali metal hydroxide such as sodium hydroxide. The alkali metal hydroxide may be added to enhance the activity of the polyhydropolyborate principal catalyst. If present, the alkali metal hydroxide is preferably present in an amount corresponding to between about 1xc3x9710xe2x88x928 and 2.5xc3x9710xe2x88x924, preferably 1xc3x9710xe2x88x927 to 10xe2x88x925 moles of alkali metal hydroxide per mole of dihydroxy aromatic compound employed.
The reaction conditions of the melt polymerization are not particularly limited and may be conducted under a wide range of operating conditions. Hence, the term xe2x80x9cmelt polymerization conditionsxe2x80x9d will be understood to mean those conditions necessary to effect reaction between the diaryl carbonate and the dihydroxy aromatic compound of the present invention. The reaction temperature is typically in the range of about 100xc2x0 C. to about 350xc2x0 C., more preferably about 180xc2x0 C. to about 310xc2x0 C. The pressure may be at atmospheric pressure, supraatmospheric pressure, or a range of pressures from atmospheric pressure to about 15 torr in the initial stages of the reaction, and at a reduced pressure at later stages, for example in the range of about 0.2 to about 15 torr. The reaction time is generally about 0.1 hours to about 10 hours.
The melt polymerization may be accomplished in one or more stages, as is known in the art with other catalysts. The principal catalyst and co-catalysts of the present invention may be added in the same stage or different stages, if the melt polymerization is conducted in more than one stage. The co-catalyst may be added at any stage, although it is preferred that it be added early in the process. The co-catalyst is preferably utilized in an amount corresponding to between about 1 and about 500 molar equivalents, based on the moles of primary catalyst I utilized.
In a further preferred embodiment, the process is conducted as a two stage process. In the first stage of this embodiment, the co-catalyst is introduced into the reaction system comprising the dihydroxy aromatic compound and the diaryl carbonate. The first stage is conducted at a temperature of 270xc2x0 C. or lower, preferably 80xc2x0 C. to 250xc2x0 C., more preferably 100xc2x0 C. to 230xc2x0 C. The duration of the first stage is preferably 0.1 to 5 hours, even more preferably 0.1 to 3 hours at a pressure from about atmospheric pressure to about 100 torr, with a nitrogen atmosphere preferred.
In a second stage, the catalyst having structure I is introduced into the product from the first stage and further polycondensation is conducted. The catalyst may be added in its entire amount in the second stage, or it may be added in batches in the second and any subsequent stages so that the total amount is within the aforementioned ranges.
It is preferable in the second and any subsequent stages of the polycondensation step for the reaction temperature to be raised while the reaction system is reduced in pressure compared to the first stage, thus bringing about a reaction between the dihydroxy aromatic compound and the diaryl carbonate. Thus there is formed initially a polycarbonate oligomer which upon further polycondensation reaction at 240xc2x0 C. to 320xc2x0 C. under reduced pressure of 5 mm Hg or less, and preferably 1 mm Hg or less, affords polycarbonate having a number average molecular weight of about 8000 daltons or greater.
If the melt polymerization is conducted in more than one stage, as noted above, it is preferable to add the co-catalyst, for example, tetramethylammonium hydroxide, tetrabutylammonium hydroxide, tetrabutylphosphonium hydroxide, or tetrabutylphosphonium acetate, in an earlier stage than the principal catalyst of the present invention. In particular, it is preferable to add the co-catalyst to the reactor before the temperature reaches 220xc2x0 C., preferably before it reaches 200xc2x0 C.
The reaction can be conducted as a batch or a continuous process. Any desired apparatus can be used for the reaction. The material and the structure of the reactor used in the present invention is not particularly limited as long as the reactor has an ordinary capability of stirring and is equipped for the removal of by-product hydroxy aromatic compound formed during the course of the polymerization. It is preferable that the reactor is capable of stirring in high viscosity conditions as the viscosity of the reaction system is increased in later stages of the reaction.
Thus, in a further embodiment, the present invention provides a method for preparing polycarbonates, which comprises the steps of
(a) heating a dihydroxy aromatic compound and a diaryl carbonate for a time period sufficient to form a melt, and thereafter introducing a catalyst composition comprising a catalytically effective amount of a polyhydropolyborate compound having structure I and a co-catalyst selected from tetraalkylammonium and tetraalkylphosphonium compounds;
(b) oligomerizing the melt mixture formed in step (a) in a reaction system comprising at least one continuous reactor in series, wherein said reactor is operated at a temperature of about 210xc2x0 C. to about 290xc2x0 C., and wherein the product from the reactor has a number average molecular weight of from about 1000 to about 5500 daltons; and
(c) polymerizing the product from step (b) in a reaction system comprising at least one continuous polymerization reactor in series, wherein said reactor is operated at a temperature of about 280xc2x0 C. to 315xc2x0 C., wherein the product from step (c) has a number average molecular weight of at least about 8000 daltons.
Additives may also be added to the polycarbonate product as long as they do not adversely affect the properties of the product. These additives include a wide range of substances that are conventionally added to the polycarbonates for a variety of purposes. Specific examples include heat stabilizers, epoxy compounds, ultraviolet absorbers, mold release agents, colorants, antistatic agents, slipping agents, anti-blocking agents, lubricants, antifogging agents, natural oils, synthetic oils, waxes, organic fillers, flame retardants, inorganic fillers and any other commonly known class of additives.
The polycarbonate obtained in accordance with the present invention may be used after being mixed with conventional additives, such as plasticizers, pigments, lubricants, mold release agents, stabilizers and organic fillers. It is also possible to blend the polycarbonate with other polymers, including but not limited to, olefin polymers such as ABS and polystyrenes, polyesters, polyacrylates, polyethersulfones, polyamides, and polyphenylene ethers.